1. Field of the Invention
The present invention relates to a device and method for driving an ink-jet head which performs printing by ejecting ink onto a printing medium, and to an ink-jet printing apparatus using the driving device.
2. Description of the Related Art
Printing apparatuses suitably used as image-output means in printers, copying machines, facsimiles, and the like record an image formed of a dot pattern on a printing medium such as paper, a plastic thin plate, cloth, or the like in accordance with given image information. The printing apparatuses are classified into an ink-jet type, a wire-dot type, a thermal type such as a thermal transfer type, a laser beam type, and the like according to their image-forming methods. Among these types, an ink-jet printing apparatus ejects ink (recording liquid), for example, in a droplet form from a discharge opening of an ink-jet head onto a printing medium, thereby printing an image on the printing medium.
An ink-jet head suitably used in such an ink-jet printing apparatus is known in which an electrothermal conversion element (discharge heater) is disposed in a channel which communicates with each discharge opening, and ink is discharged by using the expansion power of a bubble generated by heat which is produced by energizing the discharge heater (for example, a bubble-jet type, advocated by the present applicant, which discharges ink by producing film boiling in ink). This type of ink-jet head can be produced through a process similar to a semiconductor manufacturing process. For this reason, the size of the discharge heater disposed adjacent to the discharge opening or along the channel disposed on the inner side (the discharge opening and the channel will be generically named a xe2x80x9cnozzlexe2x80x9d, unless otherwise specified) can be made much smaller than that of an energy producing element which has been hitherto used to discharge ink. This enables high-density mounting of nozzles.
In an ink-jet head having multiple nozzles mounted therein, normally, discharge heaters are divided into a plurality of blocks in order to limit the number of discharge heaters to be simultaneously driven in consideration of the upper limit of the maximum power consumption, and the ink-jet head is driven block by block in a time division manner within a predetermined driving period.
A related art of such time-division driving will be described with reference to FIGS. 1 to 4.
FIG. 1A shows the correspondence between nozzles arranged in the ink-jet head, and the waveforms of signals to be applied to discharge heaters corresponding to the nozzles.
An ink-jet head 1000 shown in FIG. 1A is schematically shown, as viewed from the front side of a discharge opening. Ink is discharged from nozzles or discharge openings 1 to 12, and lands on a printing medium, thereby forming an image thereon. Recent ink-jet heads have a tendency to have 200 to 2000 nozzles mounted thereon for higher printing speed and higher image quality. Herein, the ink-jet head 1000 includes twelve nozzles for ease of explanation.
A timing chart on the right side of the ink-jet head 1000 shows the waveforms of signals to be applied to discharge heaters in the nozzles. The vertical axis represents the applied voltage. A state in which the voltage is high (H) means an energized (ON) state, and a state in which the voltage is low (L) means a non-energized (OFF) state. The horizontal axis represents the time.
For convenience, the nozzles 1 to 12 are arranged in numerical order from the top of the figure. The nozzles 1 to 12 are divided into four blocks of three. Each block includes discharge heaters to be simultaneously driven, and is driven individually. When the applied voltage is high, the discharge heater is energized, and ink is discharged by using the expansion power of a bubble generated by heat. In contrast, when the applied voltage is low, the discharge heater is not energized, and ink is not discharged. The nozzles 1 to 12 are driven in a time division manner, that is, the nozzles 1, 5, and 9 are driven at a first block time, the nozzles 2, 6, and 10 at a second block time, the nozzles 3, 7, and 11 at a third block time, and the nozzles 4, 8, and 12 at a fourth block time. As a result, the discharge openings of the first to fourth blocks sequentially perform discharge operation.
FIG. 2 is a circuit diagram of a driving circuit for such time-division driving in the related art, and FIG. 3 is an operation timing chart of the components in the driving circuit.
Referring to FIG. 2, a one-shot circuit 100 detects the rising edge of a predetermined encoder signal, and generates a one-shot pulse signal A. For example, in a so-called serial type printing apparatus, encoder signals are generated at regular intervals during a main scanning process of the ink-jet head with respect to a printing medium. The one-shot pulse signal A is supplied to a timer circuit 114 and to a one-shot circuit 102 in parallel.
The timer circuit 114 is reset by the pulse signal A, and generates signals B at regular intervals. The timer circuit 114 is connected to a shift circuit 103 and a heating pulse generating circuit 104 so that the signals B are input thereto. The signal B serves as a reference signal for a block driving period shown in FIG. 1A.
The configuration and operation of the timer circuit 114 will now be described with reference to FIGS. 4A and 4B. FIG. 4A is a circuit diagram of the timer circuit 114, and FIG. 4B is an operation timing chart thereof. Reference numerals 110, 111, 112, and 113 denote toggle flip-flops (hereinafter referred to as xe2x80x9cTFFsxe2x80x9d). A pulse to be input to the TFF 110 is a square wave having a frequency of, for example, 800 kHz. The TFF 110 inverts a pulse signal Q1 output from a terminal Q at every rising edge of the input pulse signal. In this way, the TFF can reduce the frequency to half by dividing the input signal. Since four TFFs are connected in series in FIG. 1A, an output pulse B from the last TFF 113 is a square wave of 50 kHz.
The above-described pulse signal A is supplied to a reset input terminal R of each of the TFFs 110 to 113. For this reason, the TFFs 110 to 113 are reset in response to every input of a one-shot pulse signal A, and output signals Q1, Q2, Q3, and Q4 therefrom become low. When a pulse signal having a frequency of 800 kHz is input to the TFF 110, the TFFs 110 to 113 are triggered at the falling edge of the signal A, and a signal B divided by the four TFFS 110 to 113 is output.
Referring to FIGS. 2 and 3, the one-shot circuit 102 generates a one-shot pulse signal at the falling edge of the signal B, and outputs an OR signal C between the pulse signal and the pulse signal A. The signal C is supplied to a heating-pulse generating circuit 104. On the other hand, a shift circuit 103 of a Johnson counter type outputs pulse signals QA1 to QA4 in a time division manner in response to the signal B, as shown in FIG. 3, and inputs the pulse signals to the heating-pulse generating circuit 104.
The heating-pulse generating circuit 104 generates signals for energizing the discharge heaters, and outputs the signals to a driver circuit 105. Information about the ON time of the discharge heaters for discharging ink is supplied from a microcomputer or the like (not shown) serving as a control section in the printing apparatus, and the ON time (heat pulse width) of the discharge heaters is determined on the basis of the information. As shown in FIG. 3, the heating-pulse generating circuit 104 outputs a block driving signal BL1 for a period, which is determined on the basis of the information at the rising edge of the pulse signal QA1, and supplies the signal to the driver circuit 105. Similarly, the heating-pulse generating circuit 104 outputs block driving signals BL2, BL3, and BL4 for the periods determined on the basis of the information at the rising edges of the pulse signals QA2, QA3, and QA4, respectively.
The driver circuit 105 supplies driving signals to the discharge heaters corresponding to the nozzles which are caused to discharge ink according to image information. Signals G1 to G12 (signals which determine, on the basis of the image information, whether or not discharging is performed by the nozzles) are supplied to the driver circuit 105 according to the image information, and are input from the control section (not shown). That is, the driver circuit 105 generates driving signals for the discharge heaters which are activated by the signals G1 to G12, in response to the block driving signals BL1 to BL4.
FIG. 1B shows the changes in pressure inside an ink chamber due to the driving of the discharge heaters or the discharging operation of the nozzles described above. The vertical axis represents the pressure and the horizontal axis represents the time. A broken line along the horizontal axis shows the pressure equal to the outside pressure. A part over the broken line shows that the pressure inside the ink chamber is high, and a part under the broken line shows that the pressure is low.
When it is assumed that the driving period of the entire ink-jet head is designated a discharge period, one discharge period includes a period between the beginning of a driving period assigned to the first block (a block period xe2x80x9c1xe2x80x9d in FIG. 1A) and the end of a driving period assigned to the fourth block (a block period xe2x80x9c4xe2x80x9d in FIG. 1A) (hereinafter referred to as xe2x80x9cON periodxe2x80x9d), and a period between the end of the driving period of the fourth block and the beginning of the next driving operation of the first block (hereinafter referred to as xe2x80x9cOFF periodxe2x80x9d). During the ON period, a bubble generated by heat generation of the discharge heater acts to discharge ink from the discharge opening, and simultaneously acts to push the ink back into the ink chamber of the nozzle. Therefore, the pressure inside the ink chamber increases. In contrast, during the OFF period, the pressure inside the ink chamber is decreased by a refilling operation (operation of refilling the nozzle with ink by capillary action). When the ink-jet head 1000 is continuously driven, the ON period and the OFF period are alternately established, and the pressure inside the ink chamber varies during the discharge period. This causes a pressure wave in the ink chamber.
In the method for discharging ink by applying heat energy to the ink, as in the above-described bubble-jet method, when the ink is rapidly heated by the discharge heater, water, which serves as the principal component of the ink, adjacent to the surface of the discharge heater changes state, and turns into vapor. This vapor produces a bubble, and the ink is discharged by using the expansion power of the bubble as motive power. When the discharge heater is deenergized, the bubble disappears as the vapor returns to water. However, when the temperature of the ink increases due to the continuous driving, the air in the ink cannot be dissolved in the ink, and stays as a bubble.
In general, ink discharging operation must be repeated many times in order to form an image with a lot of ink dots. One nozzle sometimes discharges ink several thousands to several ten thousands of times. Consequently, bubbles produced by the dissolved air, as described above, sometimes accumulate, grow in size to a relatively large diameter with time, and stay inside the ink chamber. In such a case, the natural frequency of a meniscus surface at the discharge opening of the nozzle (an interface between the ink and air (outside air)) decreases, and the meniscus surface tends to vibrate. When the natural frequency approaches the driving frequency, resonance is likely to occur. In a resonant state, the ink at the discharge opening is convex toward the outside of the nozzle when the pressure in the ink chamber increases, and is concave toward the inside of the nozzle when the pressure decreases. The states of the ink repeatedly changes, and the meniscus surface vibrates (hereinafter, this phenomenon will be referred to as xe2x80x9cmeniscus vibrationxe2x80x9d).
When a discharging operation is performed in such a state in which the ink at the discharge opening is convex, the amount of ink to be discharged ink is increased. Conversely, when a discharging operation is performed in a state in which the ink is concave, the amount of ink to be discharged is decreased. When the amount of ink to be discharged from the nozzle varies in such a manner, the image quality deteriorates, for example, bands appear in a formed image.
This phenomenon will be described with reference to FIG. 1C. FIG. 1C shows the sectional side of the ink-jet head, and the states of meniscus vibrations caused at the discharge openings of the nozzles. The vertical axis represents the state of a surface between the ink at the discharge opening of each nozzle, and air (meniscus surface). A state in which the meniscus surface is placed on a broken line corresponding to the discharge opening shows a normal state. As the meniscus surface becomes higher than in this state, it becomes more convex toward the outside of the discharge opening. Conversely, as the meniscus surface becomes lower than in this state, it becomes more concave toward the inside of the discharge opening.
In FIG. 1C, a bubble 1004 remains in an ink chamber 1001, as described above, and exists adjacent to the nozzle 1. At the nozzle closer to such a remaining bubble 1004, the meniscus surface is more prone to resonate, and the amplitude of the meniscus vibration is higher. In contrast, at the nozzle further apart from the bubble 1004, the meniscus surface is less prone to resonate, and the amplitude of the meniscus vibration is low. Such differences in meniscus vibration cause variations in the amount of ink discharged from the nozzles, and the discharging direction. As a result, bands are formed in a printed image due to nonuniform printing, and the image quality deteriorates.
Accordingly, the present applicant has proposed an ink-jet recording apparatus in which ink is discharged from a number of (one) discharge openings of a plurality of discharge openings in an ink-jet head, which discharges an amount of ink corresponding to 7% or less of the amount of ink discharged from all (sixty-four) the discharge openings, at the same time, and in which the total ink discharge period of all the discharge openings is set to be 70% or more of the driving period (Japanese Laid-Open Patent No. 05-084911). The above publication teaches that the amount of ink to be discharged within a unit time can be minimized, the level of the negative pressure produced in the ink chamber can be brought closest to the normal pressure, and this makes it possible to minimize the amplitude of the vibration caused in the refilling operation, to stabilize discharging, and to further increase the driving frequency.
The technique disclosed in the above publication will be described with reference to FIGS. 1A to 1C. In the publication, xe2x80x9cthe total ink discharge period is set to be 70% or more of the driving periodxe2x80x9d means that the ON period is 70% or more of the discharge period. This can be expressed by the following equation:
ON period greater than discharge periodxc3x970.7
By making such a condition, the variations in pressure in the ink chamber shown in FIG. 1B are reduced. Even when the remaining bubble 1004 shown in FIG. 1C grows, the amplitude of the meniscus vibration is decreased. That is, as the ON period further approximates the driving period, the driving frequency components in the pressure wave in the ink chamber reduced. As a result, the meniscus vibration is lessened.
However, since an operation of transferring data for discharging to the ink-jet head is performed during the OFF period, the OFF period cannot be removed. As the OFF period exists, the driving frequency component remains in the pressure wave in the above driving method. Consequently, resonance of the meniscus surface and the meniscus vibration are unavoidable. As long as the meniscus vibration occurs, the amount of ink to be discharged and the discharging direction vary depending on the ink discharging timing, as described above, and the quality of printed images is lowered.
The present invention has been made to overcome the above problems, and relates to a technique for reducing meniscus vibration in order to stabilize an ink discharging operation and to achieve high-quality printing.
According to an aspect of the present invention, there is provided an ink-jet recording apparatus for performing recording by using an ink-jet head having a plurality of discharge openings for discharging ink therefrom, and an ink chamber for supplying the ink to the discharge openings. The ink-jet recording apparatus includes a block dividing means for dividing a plurality of recording elements for discharging the ink from the discharge openings into a plurality of blocks, and driving the recording elements block by block, and a control means for driving the recording elements so that driving periods of the blocks are not equal.
According to another aspect of the present invention, there is provided an ink-jet recording apparatus for performing recording by using an ink-jet head having a plurality of discharge openings for discharging ink therefrom, and an ink chamber for supplying the ink to the discharge openings. The ink-jet recording apparatus includes a block dividing means for dividing a plurality of recording elements for discharging the ink from the discharge openings into a plurality of blocks, and driving the recording elements within predetermined driving periods, and a control means for driving the recording elements so that the time at which the driving of the first block starts varies according to the driving periods.
According to a further aspect of the present invention, there is provided a driving method for an ink-jet head having a plurality of discharge openings for discharging ink therefrom, and an ink chamber for supplying the ink to the discharge openings. The driving method includes a block dividing step of dividing a plurality of recording elements for discharging the ink from the discharge openings into a plurality of blocks, and driving the recording elements block by block, and a control step of driving the recording elements so that driving periods of the blocks are not equal.
According to a further aspect of the present invention, there is provided a driving method for an ink-jet head having a plurality of discharge openings for discharging ink therefrom, and an ink chamber for supplying the ink to the discharge openings. The driving method includes a block dividing step of dividing a plurality of recording elements for discharging the ink from the discharge openings into a plurality of blocks, and driving the recording elements within predetermined driving periods, and a control step of driving the recording elements so that the time at which the driving of the first block starts varies according to the driving periods.
According to the above structures, resonance of a meniscus surface which occurs in response to a pressure wave in the ink chamber is suppressed.
Since the meniscus vibration can be reduced by thus preventing the meniscus surface from resonating, it is possible to achieve a stable ink discharging state and to produce high-quality prints without any mottles and bands.
In this specification, xe2x80x9cprintingxe2x80x9d (sometimes referred to as xe2x80x9crecordingxe2x80x9d) broadly encompasses not only forming meaningful characters, graphics, and the like based on information, but also forming images, patterns, and the like on printing media or performing processing on printing media, whether or not the images and the like are meaningful and whether or not they are visible to the human eyes.
A xe2x80x9cprinterxe2x80x9d encompasses not only a completed apparatus for printing, but also a device which has a printing function.
A xe2x80x9cprinting mediumxe2x80x9d broadly encompasses not only paper to be used in a general type of printing apparatus, but also other materials which can receive ink, such as cloth, a plastic film, a metal plate, glass, ceramics, wood, and leather. Hereinafter, the printing medium will also be referred to as a xe2x80x9csheetxe2x80x9d or simply as xe2x80x9cpaperxe2x80x9d.
Furthermore, xe2x80x9cinkxe2x80x9d (sometimes referred to as xe2x80x9cliquidxe2x80x9d) is broadly defined herein in a manner similar to that of the above xe2x80x9cprintingxe2x80x9d, and means a liquid which is applied on a printing medium and is used to form images, patterns, and the like thereon, to process a printing medium, or to process ink (for example, to coagulate or insolubilize coloring materials in the ink applied on a printing medium).
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.